OS Gemini twin rear carb gas conversion

[mitchilito]

Just to keep the discussion rolling...
Mechanically speaking, are there any pros/cons of moving the carb to the rear of the engine?

I noticed all (or at least most) glow engines the carb intake is partially (or totally) facing the prop wash. I guess this forces air into it.
And most gas engines have the intake 90 degree to the prop wash ( or some of them the rear carb mounted as Mitchilito did here)

So... is this done to avoid changes on the prop wash to affect mixture therefore smoother runs? or is it just to move the carb weight closer to the CG and minimize the nose momentum? OR number 3 I'm overthinking this?
The only con I can think of is that requires more parts fabrication so I'd like to know the benefits since all the conversions I've seen so far keep the carb (and intake tubes in the original position (just the intake facing straight down.

Sorry if this has a simple answer, I'm not an engineer (or a machinist to be honest) I just have enough tools to get myself in trouble.
Fernando

I put the carb on back mainly as a way of saving space below the engine. In the Rascal (see pics above) I use that large space for the ignition.

I also have the bigger version of the Gemini (in my case the 300) that I plan to convert to gas. To be honest, with this bigger engine (it’s 50cc) I’m not sure where I will put the carb. I’ll be using the same WT946 carb so the carb will effectively be smaller compared to engine size with this conversion. This fact may allow it to mount UNDER the engine where the stock carb was mounted and still leave room to easily mount the engine in a tight cowl.

But not really sure yet. Stay tuned - I’ll be posting that conversion right here!

 

[ferincr]

Good to know! Looking forward to it. I'll certainly will pick more tips from your experience.
Fernando

 

[mitchilito]

Correction: the carb I’m I’m using in both Gemini 160 and 300 gas conversion is the WT 962.

 

[ferincr]

Cool! That's the same I got for the 240. So I can steal your manifold design with measurements and all

 

[mitchilito]

Go for it!!

I think that carb will be perfect for your 240

 

[Bentwings]

many two stroke thats carb all or partially out the side in the air stream face loss of the fuel air stand off. This ends up as a slimy mess on the plan and-possible flame out. There are a few tapered funnel like things that sometimes correct this I’d go outbofvthecwayvto make your new carb internal in the cowl. Be careful about pipe length. More than once I’ve had unexplained tuning issues . A good example is the zenoah GT 80z it has a slightly twisted manifold . Air especially with fuel in it does not like to turn corners . Sometime just a thicker or thinner spacer under the carb solves it. Use a plastic rather than metal spacer to keep heat away.
Just to keep the discussion rolling...
Mechanically speaking, are there any pros/cons of moving the carb to the rear of the engine?

I noticed all (or at least most) glow engines the carb intake is partially (or totally) facing the prop wash. I guess this forces air into it.
And most gas engines have the intake 90 degree to the prop wash ( or some of them the rear carb mounted as Mitchilito did here)

So... is this done to avoid changes on the prop wash to affect mixture therefore smoother runs? or is it just to move the carb weight closer to the CG and minimize the nose momentum? OR number 3 I'm overthinking this?
The only con I can think of is that requires more parts fabrication so I'd like to know the benefits since all the conversions I've seen so far keep the carb (and intake tubes in the original position (just the intake facing straight down.

Sorry if this has a simple answer, I'm not an engineer (or a machinist to be honest) I just have enough tools to get myself in trouble.
Fernando

 

[Bentwings]

Cool! That's the same I got for the 240. So I can steal your manifold design with measurements and all

Use a tachometer and temp EGT measurement for tuning . You will probably have at least half a dozen props getting props right. Keep good notes

 

[James Barker]

I somehow "lost" this post for quite sometime now and have just rediscovered it. I really enjoyed going over it and have gleaned many addition points of interest along the way. I have always had a soft spot for the OS Gemini series of engines and perhaps one day, armed with the information presented here, I will in fact have one of them pulling one of my model aircraft through the skies. Well done and thank you for sharing your findings with us.

Jim

 

[mitchilito]

Thank you for the kind words, Jim. I glad you are enjoying.
Mitch in NC

 

[ferincr]

Well, my engine arrived.
It seems like it's been sitting for a long time, I managed to remove the back cover and it looks like the rear cam bearing is shot so the best thing to do is take it completely apart. New set of bearings are already ordered.
It does need new exhaust pipes, the ones it came with have holes drilled on them (as for smoke inlets or something) so some thin wall SS tubes are also ordered. Hopefully I figure out how to form the 90 degree lips at the one end (tips are welcomed) and then experiment on bending them.

For what I saw here seems like the best way would be to fill them with something. I read somewhere else somebody used salt instead of sand (just to wash/dissolve any left over residue left imbedded in the metal).
I cannot get any bismuth or cerrobend here so I might also try filling one up with molten lead but will see...

Mitch let me know if you prefer me to stay out of your thread. I guess my posts are going to be more screams for help than helpful information
Fernando

 

[mitchilito]

Happy to have you join in, Fernando. Here's what I've learned about bending thin wall stainless tubing:

1. Anneal tubing by heating to BRIGHT ORANGE. This is not the same as plain steel which would be a dull red.
2. Fill tubing with CERROBEND. It's a low melt temp metal. It comes in several melt ranges - get the lowest. Handle it carefully - it's actually somewhat toxic :-O
3. You MUST have a tubing bender. I made my own - see pics.
4. I make the flanges with a tubing flaring tool - see pics. I first use the 45 degree bevel side of the fixture to start the flare then I put the tubing in the fixture backwards (the flat side) and use a small ball peen hammer to flatten the flare completely. This method has worked well for me for years.

 

[ferincr]

Thanks!!!! Great info!
I haven't thought of the flaring tool. My thought was to make a insert tool of steel with the shoulder and insert it on the tail end chuck and push it into the spinning tube while heating it with a torch. (a lot more complicated than your solution).
I'll see if I can get any cerrobend online and import it successfully into the Country but if it's labeled as toxic I doubt it'll make it through Customs.
As far as tube bender I ordered a cheap one (one of those scissor type) at least to give it a try. If that does not do it I'll try to fabricate something like yours. In that case there will be more questions...
Unfortunately nothing is going to happen for few weeks since I'll be travelling.

 

[petertha]

I made a similar style of tube flaring tool. This happens to be aluminum Versatube & also used on auto brake line for exhaust stacks. Sorry no experience with stainless. I made the round flaring head part removable so you could substitute another with another & still use the same frame. Or maybe if required a 2-stage squeeze for example. I found gripping the tube was a bit of trial & error. Too loose & it slides back. Too tight & it distorts. Maybe the fill alloy would solve this, I want to get some. Some of the auto brake line blocks have serrations inside (threads essentially). That's what I ended up doing (low profile tapping) & the surface was easily reconditioned with a Dremel flap wheel.

 

[petertha]

@mitchilito I meant to ask you,when you do your methanol glow to gasoline conversions, do you also alter the compression ratio?

 

[Bentwings]

. For bendingvthin wall tube you need the radius bender tool then, most important some way of preventing collapse ovthevtubebor kinking . I’ve used cerobend very low melting point metal lug the tube ends with alum foil and pour in the low melting metal . It’s available from mc master carr maybe Amazon . I used the temp range that boiling water would melt. The material is quite soft so it bends easily once in a while if you don’t get a good fill there will be a void where the tube will bend or kink . Once done you just dig out the foil and heat up some water hot enough to melt the stuff . It pours out like solder . Make sure you get all of it out . A little help from a small torch works . It will raise havoc with TIG welding or brazing if ther is any left in the tube . It doesent wet like solder but can get stuck on burrs in the tube so make sure the tube is very clean to start with. I actually use copper fittings if I can they come from 2/4” up. By weighing snd carefully trimming you can get the weight down 60-70%. Silver braze or TIG weld all copper or thin stainless. It’s much harder now with my vision issues so I don’t do much now . I instruct , those that will listen . It’s a very fussy job and time consuming . It’s hard enough building headers for full size car. Now scale to 2/4 scale . Degree of difficulty goes up by a factor of at least 10
Well, my engine arrived.
It seems like it's been sitting for a long time, I managed to remove the back cover and it looks like the rear cam bearing is shot so the best thing to do is take it completely apart. New set of bearings are already ordered.
It does need new exhaust pipes, the ones it came with have holes drilled on them (as for smoke inlets or something) so some thin wall SS tubes are also ordered. Hopefully I figure out how to form the 90 degree lips at the one end (tips are welcomed) and then experiment on bending them.

For what I saw here seems like the best way would be to fill them with something. I read somewhere else somebody used salt instead of sand (just to wash/dissolve any left over residue left imbedded in the metal).
I cannot get any bismuth or cerrobend here so I might also try filling one up with molten lead but will see...

Mitch let me know if you prefer me to stay out of your thread. I guess my posts are going to be more screams for help than helpful information
Fernando

 

[mitchilito]

@mitchilito I meant to ask you,when you do your methanol glow to gasoline conversions, do you also alter the compression ratio?

No, I did not change compression. Runs great as is. See for yourself:

 

[ferincr]

Well... I managed to get a second FT240. This one looks a lot more like it will run (at least in the pictures) So now I can experiment on the beaten one and if the worse happen I have extra parts...

 

[mitchilito]

Wow, that engine looks great! I can't wait to see your progress. I'm super busy building a 50% scale Bucker biplane or I'd start right in on mine. I'm REALLY looking forward to that.

 

[ferincr]

Thanks! I'm leaving town now for a couple of weeks, when I come back I will have the bearings to take engine #1 apart, see the inside condition and then start machining the magnet and sensor rings ( which I think would be the easier parts).

 

[ferincr]

Hello,
I thought I post what I've done so far and may be get some input to improve (not on the cosmetics I already know they look ugly and even worse on picture than to the naked eye).
Hopefully some sanding will get rid of the ugliness.
I still have to drill few holes to attach it to the engine block and the ones on the stack
One think it caught my attention last night was that I pulled out a Rotomotor engine I have (this is a 85cc already gasoline) and it comes from factory with a Walbro WT-962, so it either comes with a too small carb or the 962 is too big for the FT-240.
This morning I bumped into a YT video from CH Ignitions on that same Roto 85cc that he changed the factory ignition for a RcExl and replaced the carb.
He seems very succinct on the info given and does not mention why or what carb he replaced with (not even when somebody asked about it in the comments) so now I'm very curious.
Anyway... here are some ugly pics of some very ugly looking parts, funny you don't see all those scratches on the velocity stack other than in the pics...
Fernando